Cold cathodes are materials or structures that emit electrons with the application of electric fields without heating the emitter significantly above room temperature. Examples of cold cathodes are small metal tips with sharp points that are fabricated together with a grid structure around the tips such that an appropriate bias placed between the grid structure and the tips will extract electrons from the tips when operated in a suitable vacuum environment (Spindt emitters).
Diamond, diamond-like carbon (DLC) and other forms of carbon films have also been investigated for use as cold cathode electron emitters for many applications, such as flat panel displays, microwave device applications, backlights for liquid crystal displays (LCDs), etc. Many different techniques for growing the carbon films were tried resulting in a wide variety of carbon films. The mechanism for electron emission from these carbon films is not clear and is the subject of much investigation. What has been found consistently is that electrons are not emitted uniformly from the carbon cold cathodes, but are instead emitted from specific areas or sites of the carbon film. These areas are the emission sites (ES). The density of these sites in a unit area is referred to as the emission site density (ESD).
Researchers recognized early on that the negative electron affinity of the hydrogen terminated &lt;111&gt; and &lt;100&gt; faces of diamond may be important. A material having negative electron affinity (NEA) means that if an electron is in the conduction bands of the material, this electron has no barrier to prevent it from leaving the material if the electron diffuses to the surface having the NEA property.
The question for diamond has always been how to get an electron into the conduction band of diamond. This is not an easy question since diamond is an insulator with a very wide energy gap (5.5 eV) between the conduction band and the valence band. For an insulator at room temperature with this large a band gap, the population of electrons in the conduction band is too small to support any substantial emission current. Researchers have speculated that the electrons are injected into the diamond from a back side contact.